The present invention relates to a hydrogen removing apparatus for removing a hydrogen gas generated within a free space.
An apparatus in which a hydrogen gas is generated includes, for example, a vessel housing a nuclear reactor. FIG. 11 is a cross sectional view schematically showing a conventional vessel housing a nuclear reactor. As shown in the drawing, a nuclear reactor pressure vessel 101 housing a nuclear reactor core 107 is housed in a nuclear reactor-housing vessel 102, which is also called a primary containment vessel. The nuclear reactor-housing vessel 102 comprises an upper dry well 103, a lower dry well 104, and a wet well 105 communicating with the upper dry well 103 via a vent pipe 106 and having a suppression pool 105a arranged in a lower portion. A nuclear reactor primary cooling pipe such as a main vapor pipe 108 is connected to the nuclear reactor pressure vessel 101.
If the nuclear reactor primary cooling pipe such as the main vapor pipe 108 should be ruptured, a nuclear reactor primary cooling material having a high temperature and a high pressure is released into the upper dry well 103 within the nuclear reactor-housing vessel 102. As a result, the pressure and temperature within the upper dry well 103 are rapidly elevated. The cooling material is mixed with the gas within the upper dry well 103 and released through the vent pipe 106 into the water within the suppression pool 105a so as to be cooled by the heat energy absorption performed by the water.
In the cooling step, the water within the suppression pool 105a is poured into the nuclear reactor pressure vessel 101 by the function of an emergency reactor core cooling system so as to cool the reactor core 107. The cooling water absorbs decay heat from the reactor core 107 over a long period of time and flows into the dry wells 103, 104 through a broken port of the broken pipe. In this case, the pressure and temperature within the upper dry well 103 are kept higher than those within the wet well 105. Within the nuclear reactor of a light-water type nuclear power plant, which is put under such a long term situation, the water acting as a coolant is decomposed by radiation so as to generate a hydrogen gas and an oxygen gas.
Also, if the temperature of the fuel-covering pipe is elevated, a so-called “metal-water reaction” takes place between steam and zirconium forming the fuel-covering pipe, with the result that a hydrogen gas is generated in a short time. The hydrogen gas thus generated is released from the broken port of the broken pipe into the nuclear reactor-housing vessel. Since the hydrogen gas cannot be condensed, the pressure within the nuclear reactor-housing vessel 102 is increased with increase in the hydrogen gas concentration within the nuclear reactor-housing vessel 102.
When the hydrogen gas concentration and the oxygen gas concentration are increased to exceed a combustible limit, the gas is put in a combustible state. If the hydrogen gas is further increased, an excess reaction tends to take place.
Under the circumstances, in the conventional boiling water type nuclear power generation facilities, a nitrogen gas is substituted within the nuclear reactor-housing vessel of a pressure suppression type so as to maintain the oxygen concentration at a low level and, thus, to prevent a combustible atmosphere from being formed within the nuclear reactor-housing vessel by the hydrogen gas generated by the metal-water reaction. It is also conceivable to permit the hydrogen gas and the oxygen gas within the nuclear reactor-housing vessel to be combined again to form water by a dynamic means such as a blower connected to an external power source so as to suppress increase of the combustible gas concentration.
Japanese Patent Disclosure (Kohyo) No. 5-507553 discloses a method of statically controlling the combustible gas concentration by promoting the re-combination reaction between hydrogen and oxygen by using an oxidizing catalyst of hydrogen and without requiring an external power source.
Each of the methods described above is effective in the case where the oxygen gas has a reasonably high concentration. However, it is difficult to remove the hydrogen gas under the conditions that a large amount of a hydrogen gas is generated by the metal-water reaction and the oxygen gas concentration is low. In the present system, it is planned to release the atmosphere within the nuclear reactor-housing vessel to the outer environment so as to lower the pressure within the nuclear reactor-housing vessel and, thus, to solve the problem. In this case, however, it is possible for radioactive waste material to be discharged to the outer environment.
Under the circumstances, it is proposed to use a hydrogen absorbing alloy for removing the hydrogen gas even under the condition of a low oxygen concentration. However, the weight of the hydrogen absorbed by the hydrogen absorbing alloy is only about several percent of the alloy weight. For example, the weight of the hydrogen absorbed by a TiFe alloy widely used nowadays as a hydrogen absorbing alloy is about 1.8% of the alloy weight. Naturally, a tremendously large amount of a hydrogen absorbing alloy is required for coping with the case where a large amount of a hydrogen gas is generated as in the metal-water reaction. It follows that it is impractical to use a hydrogen absorbing alloy for removing the hydrogen gas generated in a nuclear reactor-housing vessel.